Workers in the field of illumination are aware that the art of illuminating objects (e.g. for imaging thereof) is proceeding apace with increasing need to more effectively provide intense lighting which is nonetheless diffused and uniform across a field of view, while also being cost-effective and free of harmful thermal effects.
This invention provides an illumination system that can be useful with an electronic image-capture system, giving a combined platform apt for integration in a high-speed processor. This platform provides the ability capture a video image of an object as it passes through the processor at full speed, and direct the captured image data to additional electronics which process and compress the captured data. The data may then be stored in storage and retrieval systems where it may be accessed for further manipulation and processing, displayed on video workstations, printed (typically using high-definition laser printers), transmitted electronically from point to point, placed in an archive system where it may be stored for long periods, or any combination of these processes as defined by the system parameters. Such processes, or combinations of processes, are typically controlled by mainframe computers which act as "host" systems to link multiple imaging systems, storage and retrieval systems and other peripherals.
The transport systems for which this imaging platform is contemplated are capable of transporting objects past processing stations at speeds between 100 to 400 inches per second (ips, or 2.54 to 10.16 meters per second mps). The objects, with a typical thickness of 0.005 inches (0.125 millimeters) move along a vertical constraining channel known as a "track, " having a width of typically 0.070 to 0.090 inches (1.78 to 2.29 millimeters), known as the "track gap". This "track gap" is made considerably wider than the width of the object to accommodate variations due to tolerances and also to accommodate foreign objects (e.g. documents with staples, paper-clips and the like) which inevitably are attached to such objects from time to time. While this "wide" track gap is necessary for reliable transport of documents, it creates an additional problem of depth of field for a potential video imaging system. Such system must be able to capture an image of the object e.g. a sharp focus and with correct illumination no matter where it happens to be located within the track gap. The location of the object within the track gap may also change from point to point within the length of a single object.
The field of view to be captured by an imaging system such as the one described here is largely defined by the maximum height of the objects (e.g. documents) which must be captured. The objects here contemplated (checks and like financial instruments) are typically between 2.75 to 4.75 inches (70.0 to 121.0 millimeters) high and between 6.0 and 10.0 inches (152.0 and 254.0 millimeters) long.
Image capture is achieved by passing the object in front of a photosensitive device known as a CCPD, well known to workers in the art. This consists of a large number of very small, individual photosensitive receptors, disposed in a linear array. The object is passed in front of the CCPD as a natural function of the object transport, and a complete image of the object linear segments individually captured from the CCPD, a process known in the art as "line scanning".
The photoreceptors of the CCPD consist of a semiconductor material which is formulated to convert incident light into an analogue electrical signal, which varies in potential depending upon the intensity incident light. The captured image thus consists of many individual records from each receptor, known as "pixels", each having a particular analogue potential which corresponds to the intensity of the incident light during the time the individual record was captured.
The time available for a receptor to capture this data is very short. In the preferred embodiment, and in other embodiments described e.g. in U.S. Pat. Nos. 5,063,599, 5,003,189, 5,063,461, 5,155,776, 5,144,457, 5,259,05, and 5,255,107, the document transport velocity is 300 ips (7.62 mps) and the preferred perceived size of an individual pixel is 0.005" (0.125 millimeters) square. So the time available for the individual receptor to capture the data related to particular pixel is 0.005.div.300 or approximately 17 millionths of a second (17 microseconds). At the maximum velocity (of which the preferred embodiment is capable.), this time may be as brief as 12.5 microseconds. This time is known to workers in the art as "integration time", since it describes the time available for the photosensitive receptor to gather all the photons which have struck its surface and integrate their energy to produce an analog potential (i.e. electronic image).
The CCPD of choice in the preferred embodiment is the Reticon RL1288D, a linear array device contained in the familiar "chip"-style package, which contains sufficient receptors to allow us to image said documents at the preferred pixel size and can maintain the necessary data rates (up to 80 megabytes per second) to permit imaging at the preferred document speeds. This device is commercially available and attractively priced.
It will thus be seen that the selection of the type and intensity of the document illumination system is very critical and must be closely matched to the physical parameters of the document transport and the electronic characteristics of the CCPD which is to be employed. The illumination system must provide enough light to produce adequate signal from the CCPD under the worst possible conditions for document, transport, CCPD, optics and processing electronics. The illumination system should also provide lighting which is uniform over the entire surface of the document to be imaged, in order that the CCPD may provide a consistent response for a given condition at different points on the document. The illumination must also be correctly devised to accurately render the colour and contrast of the document, since the CCPD is a monochrome device which will render images only in shades of grey. An object hereof is to teach a means of deriving such illumination.
Because of the potential archival nature of the captured image and the many possible uses to which it will be put, a very high standard of legibility must be applied. We expect to satisfy the "eyeball test"--that is, what visible on the document must be visible in the image. While later electronic processing can remove or suppress selected information, it cannot synthesize data which was not originally captured. Therefore, the illumination must be of sufficient intensity to faithfully render all significant data on the document at the same intensities and contrasts as would be observed with the human eye.
Conversely, the image should not contain data which is not visible to the human eye; this might tend to confuse or obscure the imaged data which is human-visible. For this reason, the illumination must also be spectrally correct, taking into consideration the spectral response of the CCPD (which does not necessarily mimic that of the human eye) and the effects of any optical elements used in the image camera. This combined response must approximate the response of the human eye, known to workers in the art as "photopic response." Failure to match this response may result in startling artifacts in the captured image, such as images of inks or imprints which are not visible to the human eye but are perceived by the CCPD as a result of its own spectral response, or the spectral characteristics of the illumination, or both. Such artifacts are particularly common with the increasing use of security-motivated imprints and watermarks which are invisible to the unaided eye but which will fluoresce in the visible spectrum when illuminated at extra-photopic frequencies, such as ultra-violet light.
Although the photopic response is the "baseline" for which we aim, it has been found that certain carefully-designed alterations to the spectral response of the camera system can enhance the photopic response without adding or subtracting data from the image. For example, a slight adjustment of the spectral response to the red end of the visible spectrum, combined with modifications to the response curve edge rates, will enhance contrast and legibility of documents printed in multiple similar shades of red ink. Similar adjustments may be used to enhance other specific document styles, as will be well understood by workers in the art, and may be aided by filter means (e.g. see FiL below).
One drawback of our preferred CCPD device is the size of the photosensitive array, which is of the order of 0.6 inches (15.0 millimeters) long. Since the document may be up to 4.75 inches (121 millimeters) high, an optical system, including a lens, is required to reduce the image from the height of the document to fit upon the CCPD. The optical system for capturing the image from the document thus becomes somewhat more complex, and is generally referred to as a "camera".
For reasons of stability, ease of construction and cost, we prefer to combine the illumination system and the image camera into a single unit (an IMAGER) which can be more easily integrated into an existing document processing system. Where we want to capture images of both the front and rear of a document simultaneously as it passes through the system, two such illumination/camera units are required, and we prefer to further combine both units into a single IMAGER module, which contains complete illumination systems and imaging optics/CCPDs for both front and rear faces of the document (e.g. see IMAGER embodiment of FIGS. 2-6). An important goal is to make the parts of the two illumination systems and cameras identical so far as possible.
Dust:
Workers in the art will be familiar with some of the problems encountered when integrating optical systems into high-speed document processing machinery. The area of the document track and the associated machinery to drive the documents (belts, pulleys, rollers, shafts, motors and the like) are typically laden with dust. The dust is produced by the documents themselves, which shed fibers from the friction of driving elements and from sheared and cut document edges, and from the many high-speed drive elements such as belts and rollers, which tend to shed rubber and metal particles under continual friction with driven paper (which is highly abrasive) at elevated speeds. This material is not harmful in itself, and the machinery is designed to work unaffected even with a considerable buildup. However, dust of any sort in an imaging system will rapidly compromise the quality of captured images. If a large fragment finds its way onto some part of the optical system, it will leave undesirable spots or streaks on the images, and a buildup of dust on optical surfaces will lead to a gradual decrease in transmission of images and a consequent loss of legibility or contrast in the captured image. To avoid such undesirable effects, we prefer--as an object hereof--to package the camera, illumination means and imaging optics in a single IMAGER unit which is hermetically sealed against dust.
The selection of the light source has a major impact upon the design of such an IMAGER unit, and factors such as the cost, service life, reliability and safety of a given light source have great impact upon its design. In previous work, we have favored the use of high-pressure xenon arc lamps, high-pressure tungsten-halogen incandescent lamps and multiply reentrant fluorescent lamps to address the lighting needs of particular document processors. Previous analysis had suggested that high-pressure tungsten halogen lamps were not preferable for providing sufficient light fop use at higher document speeds,--but our latest designs, coupled with improvements in the amplification and processing of signals from the CCPD, indicate that this is now feasible. Tungsten-halogen lamps of this type offer various advantages over any other light source we have investigated for these applications. They are very efficient, produce light well-matched to the desired photopic camera response, are easily handled by unskilled personnel, are widely available and very attractively priced. They are also well suited to be applied in illumination systems utilizing an "integrating," "Lambertian" cylinder to provide intense, uniform illumination. Such is an object hereof.
Data captured from an imaging camera system of the type described is increasingly useful for tasks beyond the simple matters of document viewing and archiving. Various electronic systems are now being employed to read printed and handwritten data on financial instruments such as checks, with advantages for speedy, automated handling of such documents, as will be well understood by workers in the art. If such systems are to be employed, it is highly desirable that imaging cameras of the type described produce data in a consistent format which will not vary substantially from camera to camera. Such recognition systems rely heavily on tables and databases of previous "experience" and such data has maximum value if it is all rendered from images produced to a common format. Most important among the elements of such a format are consistent magnification and pixel size from camera to camera--that is to say, cameras of a given type should render details on the same document to the same size within a very small tolerance range. We prefer to set a tolerance on magnification of .+-.2% for the entire camera system, to ensure the best response and highest efficiency from the automated reading systems presently available.
Such a tolerance may appear liberal until it is understood that commercially-available optical components have typically very lose tolerances on optical parameters such as focal length and magnification. A tolerance of .+-.5% is not uncommon on a single component such as a lens, and camera systems of the type described may contain multiple optical components, each with a significant and additive tolerance. While closer tolerances may be obtained, they are always accompanied by higher cost (typically 3.times. to 6.times. the cost of the comparable commercial lens) and by the difficulties and risks associated with the purchase of custom-made components. We have found it preferable to design our camera systems with provision to adjust various components such that the desired tolerances of magnification and pixel size may always be achieved even with optical components having much larger individual tolerances.
Centration Error
A secondary and specific problem relates to a characteristic variation in commercially-available lens assemblies suited for use in such a camera. Such lenses all exhibit, to a greater or lesser degree, a random error known as "centration" error, which may be described as a variation between the physical and optical center lines of the lens package. This error will cause similar cameras, constructed of identical parts other than their lenses, to "look" in different places. The source of this error is shown schematically in FIG. (6). It will be readily observed that a misalignment q between the mechanical axis of the lens M--M and the optical axis O--O will give rise to an error at the document face equal to (m).times.(q), where m is the magnification ratio of the camera system. In systems of this type, the magnification ratio is typically between 7 and 10, so any error of centration at the lens could be magnified by as much as 10.times. when applied at the image plane. This is obviously a highly-undesirable condition for cameras which are intended to be easily assembled and replaced, since no two cameras will render the same image of the same document. In the worst case, it is possible that a camera might fail to see the top or bottom of a maximum-height document. As with variations in magnification/focal length tolerances, lenses may be purchased in which this error is minimized, but it is never entirely removed and the incremental cost is once again great--typically up to 5.times. the cost of the comparable commercial lens, and the additional costs for the desired focal-length/magnification tolerances need to be added to this. Once again, we prefer to eliminate this error by providing selected adjustments for certain elements of the camera which allow us to adjust this error to zero.
By means of the preferred adjustments to magnification, pixel size and centration, we may produce cameras containing individual parts with a wide range of tolerances which nevertheless render the same image of the same document, as regards both image position and pixel size.
Imaging technology as a means of improving document processing is presently under consideration in the art, e.g., as disclosed in U.S. Pat. Nos. 451 619; 4 246 808; and 5,089,713. Generally, such imaging involves optically scanning documents to produce electronically encoded images which are processed electronically and stored on high-capacity storage media (such as magnetic disk drives or optical memory) for later retrieval and display. It is apparent that document imaging can provide an opportunity to reduce manual handling and manipulation of documents, since electronic images may be used in place of the actual document.